The invention relates to a fastening system, particularly for use in reattaching a skull flap, removed during brain surgery, and leaving an opening in a patient's skull, within the skull opening, and a method of reattaching the skull flap using the fastening system.
When most brain surgery is performed, it is necessary to remove a piece of skull to provide access to the brain. This is done with a hand-held, gas powered surgical tool similar to a small router. After a small hole is made in which the bit of the router is placed, the bit is then guided to cut out the piece of the skull required. The blade cuts a gap of about 1.5 mm to 2 mm wide so that the piece removed does not fit back into its hole exactly. The piece that is removed is referred to as a skull "flap".
When the brain surgery is completed, this skull flap must be reattached to the skull. There are several methods of doing this. Most commonly, a series of matching small holes is drilled in the edge of the skull and the edge of the flap. Sutures are then passed through the corresponding holes and the flap is secured back into the skull opening from which it was taken. Because the fit is not exact due to the material removed by the router, the flap always sits slightly below the surface of the skull resulting in a depressed area that is obvious through the skin.
Another common method substitutes stainless steel wire for the suture material and fewer holes are used. There is still the cosmetically objectionable depressed area resulting.
More recently, surgeons have begun to use the titanium micro plates and screws that were developed for internal fixation of facial and finger bones. While this method results in a more cosmetic result, it is extremely expensive.
All of these methods take thirty minutes to one hour of additional surgery after four to six hours of brain surgery.
There is another, newer method just beginning to be marketed in which a titanium rivet is placed inside the skull with the stem of the rivet passing between the skull and the flap. A large "pop rivet" type tool is used to force a titanium button down over the stem of the rivet, locking the flap and the skull in place between them. Three or four of these rivets and buttons are used to secure the flap in place. This method if faster than any of the other methods and less expensive than the titanium plates, but more expensive than sutures or wires.
A major disadvantage of all of the methods that use metal as a material is that the metal components create large artifacts in the CT scans and plain radiographs that are used for post surgical follow up and diagnosis.
In the early 1970's a company by the name of Codman & Shurtleff developed and marketed a similar "rivet and button" system made of soft silicone plastic. This was never a commercial success and was soon withdrawn from the market. Silicone is too soft and flexible to provide sure fixation and the buttons on the outside of the skull were so large that they made very unsightly bumps under the skin.
According to the present invention a fastening system for, and a method of, reattaching a skull flap in a skull opening, are provided which have the major advantages of the above discussed prior art systems, but without the disadvantages. The fixation provided utilizing the fastening system, and practicing the method, of the invention is at least as secure as suturing, the components of the system are radiolucent (that is they will not substantially artifact during CT scans or radiographs), it is faster than the conventional methods, it is little more expensive than suturing or wire, cosmetic results are substantially at least as good as when microplates are used, and the fastening system is easily removable if additional surgery is required. All the components of the fastening system according to the invention are preferably injection molded, and of a biologically compatible plastic that is strong enough so that only three or four of the fastener elements are needed to secure the skull flap in place, and the components are dimensioned and constructed so that they avoid potential injury to the dura mater covering the brain, and do not form unsightly bumps under the skin.
According to one aspect of the present invention a fastening system is provided comprising the following components: A fastener element of bio-compatible radiolucent plastic material comprising a substantially disc shaped head and a shank. The shank having substantially flat first and second substantially parallel surfaces, the surfaces having ratchet teeth thereon. And a substantially disc shaped lock element of bio-compatible radiolucent rigid plastic material having a through extending opening and locking teeth defining at least part of the opening, the locking teeth cooperating with the ratchet teeth to allow the shank to pass through the opening so that the head and lock element can be forced toward each other, but not allowing movement away from each other. The fastening system typically further comprises at least one weakened section of the shank at which the shank breaks off, rather than the ratchet or locking teeth, if the fastening system is overtightened. Typically, the shank, with ratchet teeth, has an operative thickness (that is at least where it extends through the gap between the skull flap and the rest of the skull during reattachment) of about 1-2 mm. The head typically has a thickness of less than about 3 mm and the lock element has a thickness of less than about 2 mm, while the shank has a length of at least about 3 cm. Where the weakened section is at least about 2 cm from the head, the shank typically has a length of at least about 2 cm past the weakened section.
While it may not be necessary for the ratchet teeth to cover all, or even a majority of, the shank, for ease of construction and to accommodate a number of different circumstances, preferably the ratchet teeth cover at least approximately the majority of the length of each surface of the shank, at least up to the weakened section. While a wide variety of plastics may be utilized, a typical plastic that is suitable is acetel plastic, such as duPont's acetal resin sold under the tradename Delrin 100, typically with a Rockwell hardness of about M94, R120 (ASTM method D785, IS02039) (or .+-.1-4%), and a flex yield strength (ASTM D790) of 99 (or .+-.1-4%).
According to another aspect of the present invention a fastening system is provided comprising: A fastener element of bio-compatible radiolucent rigid plastic material comprising a substantially disc shaped head and a shank. The shank having a plurality of ratchet teeth thereon, and a thickness of about 1-2 mm. And a substantially disc shaped lock element of bio-compatible radiolucent plastic material having a through extending opening into at least one locking tooth defining at least part of the opening, the at least one locking tooth cooperating with the ratchet teeth to allow the shank to pass through the opening so that the head and lock element can be forced toward each other, but not allowing movement away from each other. The details of the components may be as described above.
According to another aspect of the present invention a method of reattaching a skull flap, removed during brain surgery and leaving an opening in a patient's skull, the flap having an area less than the area of the opening but substantially the same shape, and using a fastening system, comprising a fastener element made of bio-compatible radiolucent rigid plastic having a shank with ratchet teeth and a substantially disc shaped lock element with an opening defined at least in part by at least one locking tooth is provided. The method comprises: (a) Placing a plurality of fastener heads, with shanks facing outwardly, in the skull opening. (b) Placing the skull flap in the skull opening so that a gap is provided between the outer periphery of the skull flap and the periphery of the skull opening, and the fastener shanks extend through the gap. (c) Placing the lock elements over the shanks. (d) For each fastener forcing the lock toward the head so that the ratchet teeth and at least one locking tooth move with respect to each other, until the head and lock element are locked together holding the skull flap in a position closing the skull opening. And (e) removing substantially all shank portions extending outwardly from the lock elements. The sequence of the steps may be varied.
Typically (a)-(e) are practiced using either three or four fastener elements and lock elements only, and the fasteners and lock elements are typically substantially the only structures holding the skull flap in the skull opening. When the shanks have a weakened section, during the practice of (d) one or more shanks break at the weakened section, and as a matter of fact the break can be specifically engineered as part of the fastening procedure. Typically the fasteners and lock elements are dimensioned, constructed, and (a)-(e) are practiced, so that the fastening heads do not injure the patient's dura mater, and so that the lock elements do not form unsightly bumps.
It is the primary object of the present invention to provide a simple, yet highly effective, fastening system for, and method of, reattaching a skull flap to a skull covering the opening from which it was removed. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.